1. Field of the Invention
The present invention relates to an electron beam exposure system. More particularly, it relates to a scanning electron beam exposure system in which the shape of an electron beam is modified.
2. Description of the Prior Art
In recent years, in a scanning electron beam exposure system, a shaped, or rectangular beam has been used to enhance the throughput of the exposure system, as compared with a round beam.
In this system, the total current of the rectangular beam increases as the cross section thereof increases, resulting in the generation of excessive electron-electron interactions or repulsive forces, that is, so-called Coulomb scattering effects. Such electron-electron interactions result not only in increased energy point distribution around the mean beam energy but also result in deterioration of peripheral sharpness due to electron displacement and disorientation.
One approach to weakening the above-mentioned electron-electron interactions is to reduce the current density of an electron beam when the cross section thereof is large. This approach, however, is disadvantageous in regard to the throughput of the exposure system since the beam current itself is also reduced. Another approach is to shorten the length of the electron optical systems, such the magnifying and demagnifying lens systems. This, however, is difficult.
In addition, in the rectangular-beam exposure system, a first rectangular beam-shaping aperture and a second rectangular beam-shaping aperture are provided, and superposition of the two apertures is modified by a deflection unit so as to form an arbitrary rectangular beam. That is, a fluctuation in the cross section of the rectangular beam generates a fluctuation in the mode of electron-electron interactions so that the point of focus of a projected image is shifted. As a result, the sharpness of the projected image is changed in accordance with the fluctuation of the cross section of the rectangular beam.